The present invention relates to a method for controlling the phase angle of the output current of a frequency converter having an intermediate d-c current link, wherein an inverter section of the frequency converter comprises six electronic switches arranged in a three-phase bridge circuit which can be addressed in pairs in six possible switching combinations, with which a primary phase angle corresponding to a natural multiple of 60.degree. el is associated, wherein each reference value for the phase angle of the output current is fed to a drive unit for the inverter, which detects the two primary phase angles immediately adjacent to the phase angle of the output current and which switches back and forth the switching combination associated with the latter alternatingly in such a manner that the phase angle of the output current agrees with the predetermined reference value.
The invention also relates to a method for controlling the phase angle of the output voltage of a frequency converter having an intermediate d-c voltage link, wherein an inverter section of the frequency converter comprises six electronic switches arranged in a three-phase bridge circuit which can be addressed in groups of three in six possible switching combinations, with which a respective primary phase angle is associated in a natural multiple of 60.degree. el and by which at least one further switching combination can be released to which a voltage-less state corresponds, where every reference value for the phase angle of the output voltage is fed to a drive unit for the inverter, which detects the two primary phase angles immediately adjacent to the phase angle of the output voltage and switches back and forth alternatingly the switching combinations associated with them in such a manner that the phase angle of the output voltage agrees with the predetermined reference value.
From DE-OS No. 31 30 672, a method of the type mentioned at the outset is known. There, the period of the alternating cycle is either kept constant (free-running operation) or is chosen so that the period represents a natural fraction of the period of the frequency converter frequency (synchronous pulsing). For the switching of the alternating clock frequency which takes place in a pulse/pause ratio depending on the phase angle, minimum pulse times must be adhered to due to the finite commutation times of the electronic switches in the output section. If the minimum pulse times are not adhered to, the corresponding pulses must be suppressed, which leads to deviations from the desired current or voltage waveform.